An image display apparatus called head-up display (HUD) principally displays information which is required for maneuver or operation in automobiles or aircraft cockpits. An automobile driver or aircraft pilot may perceive the information displayed by the HUD as if the displayed information is present in front of the windshield.
An image display apparatus called head-mount display (HMD) is worn in the same manner as ordinary vision correction glasses. A user wearing an HMD may perceive an image displayed by the HMD as if displayed information is present in a space in front of the display lens.
The HUD and the HMD both allow a user to view images through a substantially transparent part such as windshield or lens part, and therefore these image display apparatuses are called “see-through display”. Recently these image display apparatuses have been developed a lot.
For example, a driver of an automobile on which the HUD is mounted may look forward to view information required for driving with little shift of the visual line. Therefore, the HUD may provide high safety and convenience.
The HMD may consume little power to provide a user with large images. In addition, the user may view images at any location and obtain necessary information at any place any time.
The see-through display has to mix a displayed image with external light (natural light) received from environment such as landscape. For example, the HUD mounted on an automobile mixes a displayed image with external light received from the environment near the windshield by means of a combiner. Preferably, there is little light loss both for the received external light and the displayed image during the mixture of the displayed image with the received external light.
A conventional see-through display uses a volume hologram as the combiner (cf., Patent Document 1). If the hologram is used as the combiner, an image displayed by the HUD is enlarged due to a lens effect of the hologram, so that the user may view the enlarged image.
The volume hologram does not generate high-order diffracted light. Therefore, there is a low percentage loss of the externally received light due to the diffraction by the volume hologram. The volume hologram causes highly efficient diffraction for a predetermined wavelength. For example, if a laser source is used as a light source, the HUD may achieve highly efficient light utilization due to a narrow wavelength band of the laser beam.
Interference fringes are recorded in the volume hologram used for the conventional see-through display. If a laser beam having a slightly different wavelength from that of the recording light used to record the interference fringes is incident on the volume hologram, the diffraction angle is slightly shifted from a desired direction. Therefore, if the wavelength of light emitted from a light source of the see-through display having such a volume hologram differs from the wavelength of the recording light used to record the interference fringes of the volume hologram, a display position of an image may be shifted from the desired position.
A see-through display typically has several light sources. These light sources emit different light in hue from each other. The positions of images formed with the light from the light sources of the see-through display may differ from each other, hue by hue, because of factors such as the light wavelength emitted by the light sources as well as thermal changes in wavelength. If an image is formed by mixing colors, a color shift in the image becomes more visually noticeable by factors such as individual variations of the wavelengths of the light sources and thermal changes in wavelength. For example, if there is a thermal change in wavelength from the light source, diffraction efficiency may go down to cause irregular luminance distribution and color distribution in the image. In addition, the luminance of the entire image may be reduced.    Patent Document 1: JP 2007-526498 A